Thymus-dependent humoral immune responses represent the acme of adaptive immunity. Not only must T- and B lymphocytes become activated by antigen in distinct fashions, but this activation must be coordinated in space and time, processes that depend upon molecular and cellular components of the innate immune system, cognate interactions between T and B lymphocytes, and cascades of regulated cellular migrations and differentiations. These events are often controlled by single genes and identify discrete stages of immune function. Experiments proposed in this application will define three novel and significant stages in humoral immune responses recently recognized by my laboratory and result in the elucidation of the genes that control them. First, we show that a fundamental response to inflammatory antigens is the emigration of immature- and mature B lymphocytes from the bone marrow (BM) and the seeding of peripheral lymphoid tissues with recombinationally active preB cells. This emigration is effected by the interruption of chemokine signals required to retain lymphocytes in BM; TNFalpha alone is capable of interrupting these chemokine signals. Our studies will define the regulation and consequences of what seems to be physiologic control of lymphogenesis by innate immunity. Second, we show that agents that elict this unique inflammatory reaction also elicit germinal center (GC)-Iike structures in secondary lymphoid tissues even in the complete absence of T-cell help. These cell clusters contain follicular dendritic cellls (FDC) and B cells with immature pheno- types. These immature cells proliferate and express the RAG2 recombinase protein but not BCL-6, a trans- criptional repressor thought to be absolutely required for the GC response. Our proposed studies will deter- mine if these clusters represent lymphopoietic foci and if these cells are capable of supporting both V(D)J rearrangement and hypermutation. Third, we show that CD4 T cells from B-less, muMT, mice are unable to support Ig class switching and GC responses. This dysfunction is not due to a lack of tolerance to B lympho- cytes, is independent of APC type, and remedied simply transferring muMT T cells into nude hosts for 21 days. This "habituation" shows that na'fve B cells influence the response patterns of CD4 T lymphocytes in the absence of exogenous antigen. The proposed experiments will determine the B-cell cues that drive habituation and the genes responsible for restored helper activity.